FIGS. 7(a)-7(e) are sectional views illustrating process steps in a method of producing a .lambda./4-shifted diffraction grating disclosed in Japanese Published Patent Application No. Hei.3-6901. In the figures, reference numeral 1 designates a semiconductor substrate. An image reversible resist 2 is deposited on the substrate 1. A commercial image reversible resist may be employed. An intermediate layer 4 is deposited on the image reversible resist 2, followed by a cover resist 5. The intermediate layer 4 prevents mixing of the resists 2 and 5. The cover resist 5 comprises either a positive resist or a negative resist. Reference numeral 3 designates luminous flux of interference fringes for exposure of the resists.
An image reversible resist has the following property. That is, when it is developed after exposure, a portion of the resist exposed to light is removed in developing like a positive resist. However, when the resist is subjected to image reversal baking before developing, crosslinking, i.e., a composition change due to heat treatment, occurs in the portion exposed to light, and the exposed portion remains after developing.
A description is given of the production process.
Initially, as illustrated in FIG. 7(a), the image reversible resist 2 is deposited on the semiconductor substrate 1 to a thickness of from several hundreds of angstroms to one thousand angstroms and exposed by a two-beam interference fringe exposure technique. After the exposure, the intermediate layer 4 and the cover resist 5 are deposited as shown in FIG. 7(b). Thereafter, a prescribed portion of the cover resist 5 is removed by exposure and developing, and the intermediate layer 4 is partially removed using the remaining portion of the cover resist 5 as a mask. Then, the image reversible resist 2 is developed in a region where the cover resist 5 is absent (FIG. 7(c)). Thereafter, the region where the cover resist 5 is absent is exposed to light, followed by image reversal baking to reverse the property of the resist 2. After removal of the cover resist 5 and the intermediate layer 4, the remaining portion of the image reversible resist 2 is developed (FIG. 7(d)). In order to increase the etching rate of the image reversible resist 2 by a developer, after removal of the cover resist 5 and the intermediate layer 4, only the not-patterned portion of the resist 2 is exposed i.e., flooded with light and developed. Finally, using the patterned resist 2 as a mask, the substrate 1 is etched to transfer the pattern of the resist 2 on the substrate 1, followed by removal of the resist 2. As a result, a .lambda./4-shifted diffraction grating having a .lambda./4-shifted region in the center is produced (FIG. 7(e)).
In the prior art process of producing a .lambda./4-shifted diffraction grating, the deposition of the cover resist 5 and the intermediate layer 4 on the image reversible resist 2 complicates the production process.
Further, during exposure to remove a portion of the cover resist 5 in the step of FIG. 7(c), the image reversible resist 2 under the cover resist 5 is unfavorably exposed depending on the wavelength of the light used for the exposure. In the worst case, the image reversible resist 2 is completely removed when it is developed.